Nonaqueous electrolyte secondary battery

ABSTRACT

The present invention provides a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode and a nonaqueous electrolyte. The negative electrode contains an alloy having a CeNiSi 2  type crystal structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Applications No. 2003-113190, filed Apr.17, 2003; and No. 2003-336246, filed Sep. 26, 2003, the entire contentsof both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a nonaqueous electrolytesecondary battery.

[0004] 2. Description of the Related Art

[0005] In recent years, a nonaqueous electrolyte battery using lithiumas the negative electrode active material attracts attention as a highenergy density battery, and a primary battery using, for example,manganese dioxide (MnO₂), carbon fluoride [(CF₂)_(n)], or thionylchloride (SOCl₂) as the positive electrode active material are alreadyin wide use as a power source for desk-top computers, watches, and asmemory back-up batteries. Further, with progress achieved in recentyears in miniaturization and weight reduction in various electronicappliances such as VTRs and communication appliances, the demands havebeen increased for a secondary battery having a high energy density foruse as the power source for such appliances. Much research is thus beingconducted on a lithium secondary battery using lithium as the negativeelectrode active material.

[0006] Specifically, research is being conducted on a lithium secondarybattery comprising a negative electrode containing lithium, anelectrolyte selected from the group consisting of a nonaqueouselectrolysis solution and a lithium conductive solid electrolyte, and apositive electrode containing as a positive electrode active material acompound capable of carrying out a topochemical reaction with lithium.Incidentally, the nonaqueous electrolysis solution used is prepared bydissolving a lithium salt such as LiClO₄, LiBF₄ or LiAsF₆ in anonaqueous solvent such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), γ-butyrolactone (γ-BL) or tetrahydrofuran (THF). Compoundscapable of carrying out a topochemical reaction with lithium includeTiS₂, MOS₂, V₂O₅, V₆O₁₃, and MnO₂.

[0007] However, the lithium secondary battery outlined above has not yetbeen put in to practical use. It should be noted in this connection thatlithium used in the negative electrode is pulverized after the secondarybattery is repeatedly subjected to the charge-discharge operation. As aresult, the lithium is converted into a highly-reactive lithiumdendrite, which impairs the safety of the secondary battery. Also,related damage, short-circuiting and thermal runaway of the battery tendto be brought about. In addition, the charge-discharge efficiency islowered, which shortens the cycle life. Such being the situation, thelithium secondary battery outlined above has not yet been put intopractical use.

[0008] Under the circumstances, it is proposed to use a carbonaceousmaterial capable of absorbing-desorbing lithium, such as coke, a bakedresin, a carbon fiber or a vapor-grown carbon in place of the metallithium. The lithium ion secondary battery that has been commercializedin recent years comprises a negative electrode containing a carbonaceousmaterial, a positive electrode containing LiCoO₂, and a nonaqueouselectrolyte. In this lithium ion secondary battery, a furtherimprovement in the charge-discharge capacity per unit volume is requiredin accordance with the demands for the further miniaturization ofelectronic appliances and for the continuous use of the secondarybattery over a longer period of time. Such being the situation, vigorousresearch is being conducted in an attempt to develop a lithium ionsecondary battery meeting these requirements. However, a sufficientlysatisfactory result has not yet been obtained. Therefore, it isnecessary to develop a new negative electrode material forcommercializing a secondary battery having a larger capacity.

[0009] It is proposed to use an elemental metal such as aluminum (Al),silicon (Si), germanium (Ge), tin (Sn), or antimony (Sb) as a negativeelectrode material that permits obtaining a capacity larger than thatobtained by a carbonaceous material. Particularly, in the case of usingSi as a negative electrode material, it is possible to obtain a largecapacity, i.e., a capacity of 4,200 mAh per unit weight (1 g). However,in the case of using a negative electrode formed of the elemental metalexemplified above, the bond between the adjacent metal atoms is brokendue to the repetition of the absorption-desorption of Li, which leads tofine pulverization of the negative electrode, resulting in failure toobtain high charge-discharge cycle characteristics.

[0010] Under the circumstances, it is attempted to improve thecharge-discharge cycle life of the secondary battery by using as thenegative electrode material an alloy containing element T1 that does notform an alloy with lithium, such as Ni, V, Ti or Cr and element T2forming an alloy with lithium. Also, in order to suppress thepulverization of the negative electrode material, which causes thedeterioration of the cycle characteristics of the secondary battery, itis attempted to suppress the volume expansion by dispersing, forexample, a phase reactive with lithium such as an element T2 phase, anda phase that is inactive with lithium, such as an element T1 phase in anano scale, or by making the entire alloy phase amorphous.

[0011] In any of the negative electrode materials described above, analloying reaction is carried out between the negative electrode materialand lithium so as to permit lithium to be absorbed by the negativeelectrode material. The initial charging reaction is as exemplified byreaction formula (A) given below:

T1_(x)T2_(y)+Li→xT1+LiT2_(y)  (A)

[0012] The second charge-discharge reaction et seq. after the initialcharge-discharge reaction proceeds as denoted by reaction formula (B)given below:

xT1+LiT2_(y)

Li+yT2  (B)

[0013] Since the reaction in the second reaction et seq. given byreaction formula (B) does not proceed completely reversibly, Li isretained inside the alloy, and the lithium supply source is depleted ifthe charge-discharge cycle is repeated, which makes it impossible tofurther repeat the charge-discharge cycle. Incidentally, in the case ofan amorphous alloy, the reaction proceeds smoothly in the initial stage.However, the crystallization of the amorphous alloy is promoted if thecharge-discharge cycle is repeatedly carried out, with the result thatthe cycle deterioration is generated at the stage where thecrystallization is promoted.

[0014] It should also be noted that the negative electrode material thatcarries out an alloying reaction with lithium in the charging stageexhibits a high reactivity with the nonaqueous electrolyte containing anonaqueous solvent and, thus, a film of, for example, Li₂CO₃, is formedon the surface of the negative electrode as a result of the reactioncarried out between lithium contained in the negative electrode materialand the nonaqueous electrolyte. It follows that the Coulomb efficiencyis lowered during the charge-discharge cycle. Further, in the case ofusing a positive electrode active material such as LiCoO₂ as a lithiumsupply source, lithium in the supply source is depleted with progress inthe charge-discharge cycle, with the result that a clear capacitydeterioration is observed.

[0015] A nonaqueous electrolyte secondary battery comprising a negativeelectrode containing an alloy formed of at least two kinds of elements,the alloy having a hexagonal close-packed structure and a Ni₂In typestructure, is disclosed in, for example, Japanese Patent Disclosure(Kokai) No. 2001-250541. In this negative electrode, an element M¹ suchas tin or aluminum, which is capable of electrochemically carrying outan alloying reaction with lithium, is alloyed with lithium so as tocharge the secondary battery. Therefore, lithium is stored within thealloy with progress in the charge-discharge cycle so as to decrease thelithium amount contributing to the charge-discharge operation. Inaddition, this negative electrode has a high reactivity with thenonaqueous electrolyte and, thus, the Coulomb efficiency is low duringthe charge-discharge cycle. It follows that the secondary batterydisclosed in the prior art quoted above is incapable of obtaining a longcharge-discharge cycle life.

BRIEF SUMMARY OF THE INVENTION

[0016] An object of the present invention is to provide a nonaqueouselectrolyte secondary battery excellent in both the charge-dischargecycle life and the discharge capacity per unit volume.

[0017] According to a first aspect of the present invention, there isprovided a nonaqueous electrolyte secondary battery comprising:

[0018] a positive electrode;

[0019] a negative electrode containing an alloy having a TiNiSi typecrystal structure; and

[0020] a nonaqueous electrolyte.

[0021] According to a second aspect of the present invention, there isprovided a nonaqueous electrolyte secondary battery comprising:

[0022] a positive electrode;

[0023] a negative electrode containing an alloy having a ZrBeSi typecrystal structure; and

[0024] a nonaqueous electrolyte.

[0025] Further, according to a third aspect of the present invention,there is provided a nonaqueous electrolyte secondary battery comprising:

[0026] a positive electrode;

[0027] a negative electrode containing an alloy having a CeNiSi₂ typecrystal structure; and

[0028] a nonaqueous electrolyte.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0029]FIG. 1 schematically exemplifies the construction of the unit cellof a TiNiSi type crystal;

[0030]FIG. 2 schematically exemplifies the construction of the unit cellof a ZrBeSi type crystal;

[0031]FIG. 3 is a cross sectional view schematically showing theconstruction of a cylindrical nonaqueous electrolyte secondary batteryaccording to one embodiment of the nonaqueous electrolyte secondarybattery of the present invention;

[0032]FIG. 4 is an oblique view, partly broken away, schematicallyshowing the construction of a thin plate type nonaqueous electrolytesecondary battery according to another embodiment of the nonaqueouselectrolyte secondary battery of the present invention;

[0033]FIG. 5 is a graph showing the X-ray diffraction patterns of thenegative electrode active material under the initial state, the chargedstate and the discharged state in respect of the nonaqueous electrolytesecondary battery for Example 5 and the X-ray diffraction pattern of thenegative electrode active material under the charged state in respect ofthe nonaqueous electrolyte secondary battery for Comparative Example 3;

[0034]FIG. 6 is a graph showing the X-ray diffraction patterns of thenegative electrode active material under the initial state (before thetest), the charged state and the discharged state in respect of thenonaqueous electrolyte secondary battery for Comparative Example 6 andthe X-ray diffraction pattern of the negative electrode active materialunder the charged state in respect of the nonaqueous electrolytesecondary battery for Comparative Example 3;

[0035]FIG. 7 is a graph showing the X-ray diffraction patterns of thenegative electrode active material after the discharge in the 50^(th)cycle and after the charging in the 51^(st) cycle in respect of thenonaqueous electrolyte secondary battery for Example 5 and the X-raydiffraction pattern of the negative electrode active material under thecharged state in respect of the nonaqueous electrolyte secondary batteryfor Comparative Example 3;

[0036]FIG. 8 is a graph showing the X-ray diffraction patterns of thenegative electrode active material after the discharge in the 10^(th)cycle and in the 50^(th) cycle in respect of the nonaqueous electrolytesecondary battery for Comparative Example 6 and the X-ray diffractionpattern of the negative electrode active material under the chargedstate in respect of the nonaqueous electrolyte secondary battery forComparative Example 3;

[0037]FIG. 9 schematically exemplifies the construction of the unit cellof a CeNiSi₂ type crystal; and

[0038]FIG. 10 is a graph showing the X-ray diffraction patterns of thenegative electrode active material under the initial state, the chargedstate and the discharged state in respect of the nonaqueous electrolytesecondary battery for Example 22.

DETAILED DESCRIPTION OF THE INVENTION

[0039] A nonaqueous electrolyte secondary battery according to a firstembodiment of the present invention will now be described. Thenonaqueous electrolyte secondary battery comprises a positive electrode,a negative electrode containing an alloy having a TiNiSi type crystalstructure, and a nonaqueous electrolyte layer provided between thepositive electrode and the negative electrode.

[0040] The negative electrode, the positive electrode and the nonaqueouselectrolyte layer included in the nonaqueous electrolyte secondarybattery will now be described.

[0041] 1) Negative Electrode

[0042]FIG. 1 schematically shows the construction of the unit cell ofthe TiNiSi type crystal, covering the case where the alloy has an LaNiSncomposition. Specifically, the circles shaded with oblique lines, whichare shown in FIG. 1, denote La sites, the white circles denote Ni sites,and the circles shaded with dots denote Sn sites.

[0043] As shown in FIG. 1 mentioned above, the TiNiSi type crystalstructure covers not only the case where it is made basically of thethree types of elements Ti, Ni and Si, but also the case where, as longas its crystal structure is maintained, the basic elements Ti, Ni and Siare substituted by different types of elements such as Ln, M or Sn.

[0044] It is possible for the alloy to be a single phase alloyconsisting of a TiNiSi type crystal phase or to be a polyphase alloyincluding a TiNiSi type crystal phase and another crystal phase.

[0045] The crystal axis b of the TiNiSi type crystal is the crystal axisparallel to the depth direction in FIG. 1. Lithium is inserted into thefree space between the adjacent layers in a direction perpendicular tocrystal axis b. It is desirable for the lattice constant of crystal axisb to fall within a range of 4 Å to 5.5 Å. If the lattice constant ofcrystal axis b is smaller than 4 Å, it is difficult to interpose thelithium ions between the adjacent layers of the crystal. On the otherhand, if the lattice constant of crystal axis b exceeds 5.5 Å, theTiNiSi type crystal phase possibly fails to be obtained. It is moredesirable for the lattice constant of crystal axis b to fall within arange of 4.2 Å to 5.3 Å.

[0046] It is desirable for the elements constituting the alloy toinclude Sn because this will enable the secondary battery to exhibit ahigher discharge capacity per unit volume. It is more desirable for theelements constituting the alloy to include at least one kind of element,Ln, selected from the elements having an atomic radius falling within arange of 1.6×10⁻¹⁰ m to 2.2×10⁻¹⁰ m, in addition to Sn. If the alloycontains both Sn and Ln, it is possible for the absorbing-desorbingreaction of lithium to be carried out-more-smoothly. It is also possibleto increase the stability of the crystal so as to permit the latticeconstant of crystal axis b to fall within a range of 4 Å to 5.5 Å.

[0047] The composition of the alloy is not particularly limited as longas the alloy includes the TiNiSi type crystal phase. However, it isdesirable for the alloy to have a composition represented by formula (1)given below:

(Ni_(1-(X+Z))Ln_(X)M_(Z))ySn_(100-y)  (1)

[0048] where Ln denotes at least one kind of element selected from theelements having an atomic radius falling within a range of 1.6×10⁻¹⁰ mto 2.2×10⁻¹⁰ m, M is at least one element selected from the groupconsisting of Ti, V, Co, Fe and Nb, and x, y and z satisfy theconditions of 0.4≦x+z≦0.7, 40≦y≦80 and 0≦z≦0.2.

[0049] As shown in formula (1) given above, the alloy used in thepresent invention contains at least one kind of element Ln selected fromthe elements having an atomic radius falling within a range of 1.6×10⁻¹⁰m to 2.2×10⁻¹⁰ m. Use of Ln permits the lithium ions to be interposedeasily between the adjacent layers of the crystal. In the case of usingan element having an atomic radius, which exceeds 2.2×10⁻¹⁰ m or whichis smaller than 1.6×10⁻¹⁰ m, as element Ln, it is difficult to maintainthe TiNiSi type crystal structure or it is difficult to interpose thelithium ions between the adjacent layers of the crystal.

[0050] The elements Ln that can be used desirably in the presentinvention include, for example, La having an atomic radius of 1.88×10⁻¹⁰m, Ce having an atomic radius of 1.83×10⁻¹⁰ m, Pr having an atomicradius of 1.83×10⁻¹⁰ m, Nd having an atomic radius of 1.82×10⁻¹⁰ m, Pmhaving an atomic radius of 1.80×10⁻¹⁰ m, Sm having an atomic radius of1.79×10⁻¹⁰ m, Mg having an atomic radius of 1.60×10⁻¹⁰ m, Ca having anatomic radius of 1.97×10⁻¹⁰ m, Sr having an atomic radius of 2.15×10⁻¹⁰m, Ba having an atomic radius of 2.18×10⁻¹⁰ m, Y having an atomic radiusof 1.82×10⁻¹⁰ m, Zr having an atomic radius of 1.62×10⁻¹⁰ m, and Hfhaving an atomic radius of 1.60×10⁻¹⁰ m.

[0051] If the sum atomic ratio (x+z) of the element Ln and the element Mis smaller than 0.4, it is difficult to interpose the lithium ionsbetween the adjacent layers of the crystal, possibly resulting infailure to obtain a high charging capacity. On the other hand, if thesum atomic ratio (x+z) exceeds 0.7, a phase such as a LnSn phase, whichperforms easily an alloying reaction with lithium, is formed in additionto the TiNiSi type crystal structure, with the result that thecharge-discharge cycle life of the secondary battery tends to beshortened. It is more desirable for the sum atomic ratio (x+z) to fallwithin a range of 0.45 to 0.65.

[0052] It is also possible to permit at least one kind of element, M,selected from the group consisting of Ti, V, Co, Fe and Nb to becontained in the alloy. Where the alloy contains at least one kind ofelement M, it is possible to stabilize the crystal structure and toprolong the charge-discharge cycle life of the secondary battery. Itshould be noted, however, that, if the addition amount z of element Mexceeds 0.2, it is difficult to maintain the crystal structure, whichmay cause a reduction in the charge-discharge capacity or thecharge-discharge cycle life. It is more desirable for the additionamount z of the element M to fall within a range of 0 to 0.15.

[0053] The sum atomic ratio y, i.e., the atomic ratio of the sum of Ni,the element Ln and the element M, is defined in the present invention tofall within a range of 40 to 80, as shown in formula (1). If the sumatomic ratio y noted above is smaller than 40, the Sn single phase isprecipitated, with the result that the pulverization of the alloy tendsto be promoted, which shortens the charge-discharge cycle life of thesecondary battery. On the other hand, if the sum atomic ratio y exceeds80, the alloy fails to have the TiNiSi type crystal structure such that,for example, the Ni₃Sn₂ phase that is quite inactive to lithiumconstitutes the principal phase of the crystal. It follows that thecharge-discharge characteristics or the charge-discharge capacity of thesecondary battery would be lowered. It is more desirable for the sumatomic ratio y to fall within a range of 45 to 75.

[0054] In the alloy having the TiNiSi type crystal structure, it ispossible for a part of the constituting elements to be replaced byanother element in order to impart a local strain to the crystalstructure or change the Fermi level of alloy.

[0055] An alloy having the TiNiSi type crystal structure can bemanufactured by, for example, a rapid solidification method. In therapid solidification method, the raw alloy materials weighed in advanceare melted within a crucible in an inert gas atmosphere, followed byspraying the resultant alloy melt onto a cooled body rotated at a highspeed so as to obtain a flake-like sample having a thickness of 10 to 50μm. It is possible to apply a heat treatment to the obtained sample soas to homogenize the texture and the composition of the sample.

[0056] The negative electrode is prepared by dispersing in a suitablesolvent a negative electrode mixture including a negative electrodeactive material containing an alloy having, for example, the TiNiSi typecrystal structure, an electrically conductive agent, and a binder so asto obtain a dispersion, followed by coating one surface or both surfacesof a current collector with the resultant suspension and subsequentlydrying the coating and, as required, applying a pressing to the driedcoating.

[0057] Also, in the case of using as the negative electrode activematerial a mixture containing the alloy described above and acarbonaceous material having a high absorption capability of an alkalimetal, it is possible to improve the absorption capability of the alkalimetal such as lithium. It is desirable to use a graphitized material,e.g., a mesophase pitch carbon fiber (MCF), as the carbonaceous materialused for preparing the negative electrode active material.

[0058] Further, it is possible to use a carbonaceous material as theelectrically conductive agent contained in the negative electrode. If acarbonaceous material having a high absorption capability of the alkalimetal and a high electrical conductivity is used as the carbonaceousmaterial contained in the negative electrode, it is possible for thecarbonaceous material to act also as the electrically conductive agent.If graphitized material, which has a high alkali metal absorptioncapability, such as mesophase pitch carbon fiber, is used singly as acarbonaceous material for the negative electrode active material, theelectrical conductivity of the negative electrode tends to be lowered.Such being the situation, it is desirable to use a carbon material suchas acetylene black or carbon black as an electrically conductive agenttogether with the graphitized material noted above.

[0059] The binder used in the present invention includes, for example,polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), afluorinated rubber, a styrene-butadiene rubber (SBR), and carboxy methylcellulose (CMC).

[0060] Concerning the mixing ratio of the negative electrode activematerial, the electrically conductive agent and the binder, it isdesirable for the negative electrode active material to be mixed in anamount of 70 to 95% by weight, for the conductive agent to be mixed inan amount of 0 to 25% by weight, and for the binder to be mixed in anamount of 2 to 10% by weight.

[0061] The current collector used for the negative electrode is notparticularly limited as long as the current collector is formed of anelectrically conductive material. For example, it is possible to use afoil, a mesh, a punched metal or a lath metal made of copper, stainlesssteel or nickel.

[0062] 2) Positive Electrode

[0063] The positive electrode includes a current collector and apositive electrode active material layer formed on one surface or bothsurfaces of the current collector.

[0064] The positive electrode can be prepared by, for example,dispersing in a suitable solvent a positive electrode active material,an electrically conductive agent, and a binder so as to obtain asuspension, followed by coating the surface of the current collectorwith the resultant suspension and subsequently drying the coating andpressing the dried coating.

[0065] The positive electrode active material used in the presentinvention is not particularly limited as long as the substance iscapable of absorbing the alkali metal in the discharging stage of thesecondary battery and desorbing the absorbed alkali metal in thecharging stage of the secondary battery.

[0066] To be more specific, the positive electrode active material canbe provided by various oxides and sulfides including, for example,manganese oxide (MnO₂), lithium manganese-containing complex oxides suchas LiMn₂O₄ and LiMnO₂, lithium nickel-containing complex oxides such asLiNiO₂, lithium cobalt-containing complex oxides such as LiCoO₂, lithiumnickel cobalt-containing complex oxides such as LiNi_(1-x)CO_(x)O₂,lithium manganese cobalt-containing complex oxides such asLiMn_(x)Co_(1-x)O₂, and vanadium oxides such as V₂O₅. It is alsopossible for the positive electrode active material to be provided by anorganic material such as an electrically conductive polymer material ora disulfide series polymer material.

[0067] It is more desirable for the positive electrode active materialto be provided by a material that permits increasing the batteryvoltage, such as a lithium manganese-containing complex oxide, e.g.,LiMn₂O₄, a lithium nickel-containing complex oxide, e.g., LiNiO₂, alithium cobalt-containing complex oxide, e.g., LiCoO₂, a lithium nickelcobalt-containing complex oxide, e.g., LiNi_(0.8)CO_(0.2)O₂, and alithium manganese cobalt-containing complex oxide, e.g.,LiMn_(x)Co_(1-x)O₂.

[0068] The current collector used in the present invention is notparticularly limited as long as the current collector is formed of anelectrically conductive material. Particularly, it is desirable for thecurrent collector included in the positive electrode to be formed of amaterial that is unlikely to be oxidized during the battery reaction.For example, it is desirable to use aluminum, stainless steel ortitanium.

[0069] The electrically conductive agent used in the present inventionincludes, for example, an acetylene black, a carbon black and graphite.

[0070] The binder used in the present invention includes, for example,polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF) and afluorinated rubber.

[0071] Regarding the mixing ratio of the positive electrode activematerial, the electrically conductive agent and the binder, it isdesirable for the positive electrode active material to be mixed in anamount of 80 to 95% by weight, for the conductive agent to be mixed inan amount of 3 to 20% by weight, and for the binder to be mixed in anamount of 2 to 7% by weight.

[0072] 3) Nonaqueous Electrolyte Layer

[0073] The nonaqueous electrolyte layer serves to impart an ionicconductivity between the negative electrode and the positive electrode.

[0074] It is possible to use a nonaqueous electrolyte layer prepared byallowing a nonaqueous electrolyte solution, which is prepared bydissolving an electrolyte in a nonaqueous solvent, to be supported by aseparator formed of a porous material.

[0075] The separator serves to hold the nonaqueous electrolysis solutionand to achieve an electrical insulation between the positive electrodeand the negative electrode. The separator used in the present inventionis not particularly limited as long as the separator is formed of aninsulating material and permits the ion migration between the positiveelectrode and the negative electrode. For example, it is possible to usea synthetic resin unwoven fabric, a polyethylene porous film or apolypropylene porous film for forming the separator.

[0076] The nonaqueous solvent used in the present invention includes,for example, a nonaqueous solvent containing as a main component acyclic carbonate such as ethylene carbonate (EC) or propylene carbonate(PC) and another nonaqueous solvent consisting mainly of a mixed solventcontaining a cyclic carbonate and a nonaqueous solvent having aviscosity lower than that of the cyclic carbonate.

[0077] The nonaqueous solvent having a low viscosity noted aboveincludes, for example, a linear carbonate, y-butyrolactone,acetonitrile, methyl propionate, ethyl propionate, a cyclic ether, and alinear ether. The linear carbonate noted above includes, for example,dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate. Also,the cyclic ether noted above includes, for example, tetrahydrofuran and2-methyl tetrahydrofuran. On the other hand, the linear ether notedabove includes, for example, dimethoxy ethane and diethoxy ethane.

[0078] The electrolyte used in the present invention includes, forexample, lithium hexafluoro phosphate (LiPF₆), lithium tetrafluoroborate (LiBF₄), lithium hexafluoro arsenate (LiAsF₆), lithiumperchlorate (LiClO₄), and lithium trifluoro metasulfonate (LiCF₃SO₃).Particularly, it is desirable to use at least one electrolyte of lithiumhexafluoro phosphate (LiPF₆) and lithium tetrafluoro borate (LiBF₄).

[0079] It is desirable for the electrolyte to be dissolved in thenonaqueous solvent in an amount of 0.5 to 2 mol/L.

[0080] It is possible to use a gel-like material, which is prepared byallowing a polymer material to contain a nonaqueous electrolysissolution, in the nonaqueous electrolyte layer. To be more specific, itis possible to arrange an electrolyte layer formed of the gel-likematerial between the positive electrode and the negative electrode. Itis also possible to use as the electrolyte layer the separator holdingthe gel-like material.

[0081] The polymer material used in the present invention for preparingthe gel-like material includes, for example, a monomer or polymer ofpolyacrylonitrile, polyacrylate, polyvinylidene fluoride (PVdF) orpolyethylene oxide (PEO), or a copolymer between any of these monomersand another monomer.

[0082] It is also possible to use as the nonaqueous electrolyte layer asolid polymer electrolyte layer prepared by dissolving an electrolyte inthe polymer material, followed by solidifying the resultant solution.The polymer material used in the present invention for preparing thesolid polymer electrolyte layer includes, for example, a monomer orpolymer of polyacrylonitrile, polyvinylidene fluoride (PVdF), orpolyethylene oxide (PEO), or a copolymer between any of these monomersand another monomer. It is also possible to use an inorganic solidelectrolyte for forming the nonaqueous electrolyte layer. The inorganicsolid electrolyte used in the present invention includes, for example, aceramic material containing lithium. To be more specific, the inorganicsolid electrolyte noted above includes, for example, Li₃N,Li₃PO₄—Li₂S—SiS₂, and LiI—Li₂S—SiS₂ glass.

[0083] The nonaqueous electrolyte secondary battery according to thefirst embodiment of the present invention described above comprises anegative electrode containing an alloy having a TiNiSi type crystalstructure. The alloy of the particular crystal structure does notperform an alloying reaction with lithium when lithium ions areinterposed between the adjacent layers of the crystal in the chargingstage. Also, the interposed lithium ions are released from between theadjacent layers of the crystal in the discharge stage. It follows thatlithium can be intercalated into and deintercalated from between theadjacent layers of the crystal without bringing about the alloyingreaction between the alloy and lithium in the charge-discharge stages soas to enhance the reversibility of the intercalation/deintercalationreaction of lithium. It is also possible to increase the stability ofthe crystal because the volume expansion can be suppressed in thelithium insertion stage. Further, it is possible to suppress thereaction between the negative electrode and the nonaqueous electrolyte.It follows that the Coulomb efficiency can be increased during thecharge-discharge cycle so as to realize high charge-discharge cyclecharacteristics.

[0084] It should also be noted that, in the present invention, thelattice constant of the crystal axis b of the alloy having the TiNiSitype crystal structure is set to fall within a range of 4 Å to 5.5 Å soas to carry out smoothly the intercalation/deintercalation reaction oflithium. It follows that the charge-discharge cycle characteristics ofthe secondary battery can be further improved.

[0085] The alloy having the TiNiSi type crystal structure has acomposition represented by formula (1) given previously. In this case,the alloy density can be increased to a high level, i.e., can beincreased to 7.8 g/cm³ on the average, so as to make it possible tofurther increase the capacity per unit volume. It follows that it ispossible to provide a secondary battery excellent in the capacity perunit volume and in the charge-discharge cycle characteristics.

[0086] A nonaqueous electrolyte secondary battery according to a secondembodiment of the present invention will now be described. Thenonaqueous electrolyte secondary battery comprises a positive electrode,a negative electrode containing an alloy having a ZrBeSi type crystalstructure, and a nonaqueous electrolyte layer provided between thepositive electrode and the negative electrode. The positive electrodeand the nonaqueous electrolyte layer similar to those describedpreviously in conjunction with the first embodiment can be used in thesecond embodiment. Such being the situation, the negative electrodeincluded in the nonaqueous electrolyte secondary battery according tothe second embodiment of the present invention will now be described.

[0087]FIG. 2 schematically shows the construction of the unit cell ofthe ZrBeSi type crystal in the case where the alloy has a LaNiSncomposition. Specifically, the circles shaded with oblique lines, whichare shown in FIG. 2, denote La sites, the white circles denote Ni sites,and the circles shaded with dots denote Sn sites.

[0088] As shown in FIG. 2 mentioned above, the ZrBeSi type crystalstructure covers not only the case where it is made basically of thethree types of elements Zr, Be and Si, but also the case where, as longas its crystal structure is maintained, the basic elements Zr, Be and Siare substituted by different types of elements such as Sn and rare earthelements.

[0089] It is possible for the alloy to be a single phase alloyconsisting of a ZrBeSi type crystal phase or to be a polyphase alloyincluding a ZrBeSi type crystal phase and another crystal phase.

[0090] The crystal axis “a” of the ZrBeSi type crystal is the crystalaxis parallel to the depth direction in FIG. 2. Lithium is interposedbetween the adjacent layers in a direction perpendicular to the crystalaxis “a”. It is desirable for the lattice constant of the crystal axis“a” to fall within a range of 4 Å to 5.5 Å. If the lattice constant ofthe crystal axis “a” is smaller than 4 Å, the interposition of thelithium ions between the adjacent layers of the crystal tends to berendered difficult. On the other hand, if the lattice constant of thecrystal axis “a” exceeds 5.5 Å, it is possible for the ZrBeSi typecrystal phase to fail to be obtained. It is more desirable for thelattice constant of the crystal axis “a” to fall within a range of 4.2 Åto 5.3 Å.

[0091] It is possible to prepare an alloy having the ZrBeSi type crystalstructure by, for example, a induction heating method. In the inductionheating method, it comprises pouring a melt onto a rotating coolingplate in the casting stage. It is possible to control the thickness ofthe deposited melt by adjusting the melt supply rate and the movingspeed of the cooling plate, thereby controlling the cooling rate. A heattreatment is applied to the obtained sample so as to homogenize thetexture and the composition of the alloy.

[0092] The negative electrode can be prepared by a method similar tothat described previously in conjunction with the first embodiment.

[0093] The nonaqueous electrolyte secondary battery of the secondembodiment of the present invention described above comprises a negativeelectrode containing an alloy having a ZrBeSi type crystal structure.The alloy of the particular crystal structure makes it possible tointercalate or deintercalate lithium into or out of between the adjacentlayers of the crystal without bringing about an alloying reactionbetween the alloy and lithium. As a result, the reversibility of theintercalation/deintercalation of lithium can be enhanced. It is alsopossible to increase the stability of the crystal because the volumeexpansion can be suppressed in the lithium insertion stage. Further, itis possible to suppress the reaction between the negative electrode andthe nonaqueous electrolyte. It follows that the Coulomb efficiency canbe increased during the charge-discharge cycle so as to realize highcharge-discharge cycle characteristics.

[0094] It should also be noted that, in the present invention, thelattice constant of the crystal axis “a” of the alloy having the ZrBeSitype crystal structure is set to fall within a range of 4 Å to 5.5 Å soas to carry out smoothly the intercalation/deintercalation reaction oflithium. It follows that the charge-discharge cycle characteristics ofthe secondary battery can be further improved.

[0095] As described above, the alloy having any of the TiNiSi typecrystal structure or the ZrBeSi type crystal structure makes it possibleto permit lithium to be intercalated into and deintercalated frombetween the adjacent layers of the crystal without bringing about thealloying reaction between the alloy and lithium in the charge-dischargestages. It is considered reasonable to understand that the particularfunction can be achieved as follows.

[0096] As pointed out in Japanese Patent Disclosure No. 2001-250541referred to previously, different elements M¹ and M² are alternatelypresent on a plane constituting the same layer in the Ni₂In type crystalstructure. As a result, the agglomeration of M¹ atoms caused by thealloying reaction with lithium is suppressed so as to stabilize thestructure. On the other hand, the TiNiSi type crystal structure or theZrBeSi type crystal structure of the alloy used in the first and secondembodiments of the present invention is featured in that two kinds ofatoms other than Ln are alternately bonded with each other so as to forma hexagonal plane such as graphite. In this case, it is consideredreasonable to understand that there is an electron orbit that permitselectrons to be given to and received from the lithium ion in adirection perpendicular to the formed hexagonal plane. In addition,since the coupling between the adjacent atoms is more stable than thatof the Ni₂In type crystal structure, it is possible for theintercalation reaction rather than the alloying reaction to take place.

[0097] A nonaqueous electrolyte secondary battery according to a thirdembodiment of the present invention will now be described. Thenonaqueous electrolyte secondary battery comprises a positive electrode,a negative electrode containing an alloy having a CeNiSi₂ type crystalstructure, and a nonaqueous electrolyte layer provided between thepositive electrode and the negative electrode. The positive electrodeand the nonaqueous electrolyte layer similar to those describedpreviously in conjunction with the first embodiment can be used in thethird embodiment. Such being the situation, the negative electrodeincluded in the nonaqueous electrolyte secondary battery according tothe third embodiment of the present invention will now be described.

[0098]FIG. 9 schematically shows the construction of the CeNiSi₂ typecrystal. Specifically, the circles shaded with oblique lines, which areshown in FIG. 9, denote La sites, the white circles denote Ni sites, andthe circles shaded with dots denote Si sites. Also, the regionsurrounded by a solid line denotes a unit cell of the CeNiSi₂ typecrystal. Further, crystal axes a, b, c are as shown in FIG. 9.

[0099] As shown in FIG. 9 mentioned above, the CeNiSi₂ type crystalstructure covers not only the case where it is made basically of thethree types of elements Ce, Ni and Si, but also the case where, as longas its crystal structure is maintained, the basic elements Ce, Ni and Siare substituted by different types of elements such as Ln, M1 or M2.

[0100] It is possible for the alloy to be a single phase alloyconsisting of a CeNiSi₂ type crystal phase or to be a polyphase alloyincluding a CeNiSi₂ type crystal phase and another crystal phase.

[0101] It is desirable for the lattice constant of the crystal axis “a”to fall within a range of 3.5 Å to 5.5 Å. If the lattice constant of thecrystal axis “a” is smaller than 3.5 Å, it is difficult to interpose thelithium ions between the adjacent layers of the crystal. On the otherhand, if the lattice constant of the crystal axis “a” exceeds 5.5 Å, itis possibly difficult to obtain the CeNiSi₂ type crystal phase. It ismore desirable for the lattice constant of the crystal axis “a” to fallwithin a range of 4 Å to 5 Å.

[0102] It is desirable for the alloy to contain at least one additionalelement selected from the group consisting of P, Si, Ge, Sn and Sbbecause the discharge capacity per unit volume of the secondary batterycan be increased in the case where the alloy contains the additionalelement.

[0103] It is desirable for the alloy to have a composition representedby formula (2) given below:

LnM1_(x)M2_(y)  (2)

[0104] where Ln denotes at least one kind of element selected from theelements having an atomic radius falling within a range of 1.6×10⁻¹⁰ mto 2.2×10⁻¹⁰ m, M1 is at least one element selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Nb, M2 is at leastone element selected from the group consisting of P, Si, Ge, Sn and Sb,and x and y satisfy the conditions of 0.5≦x≦1.5 and 1.5≦y≦3.5.

[0105] As shown in formula (2) given above, the alloy used in oneembodiment of the present invention contains at least one kind ofelement, Ln, selected from the elements having an atomic radius fallingwithin a range of 1.6×10⁻¹⁰ m to 2.2×10⁻¹⁰ m. Use of Ln permits thelithium ions to be interposed easily into between the adjacent layers ofthe crystal. In the case of using an element having an atomic radius,which exceeds 2.2×10⁻¹⁰ m or which is smaller than 1.6×10⁻¹⁰ m, aselement Ln, it is difficult to maintain the CeNiSi₂ type crystalstructure or it is difficult to interpose the lithium ions between theadjacent layers of the crystal.

[0106] The elements Ln that can be used desirably in one embodiment ofthe present invention include, for example, La having an atomic radiusof 1.88×10⁻¹⁰ m, Ce having an atomic radius of 1.83×10⁻¹⁰ m, Pr havingan atomic radius of 1.83×10⁻¹⁰ m, Nd having an atomic radius of1.82×10⁻¹⁰ m, Pm having an atomic radius of 1.80×10⁻¹⁰ m, Sm having anatomic radius of 1.79×10⁻¹⁰ m, Mg having an atomic radius of 1.60×10⁻¹⁰m, Ca having an atomic radius of 1.97×10⁻¹⁰ m, Sr having an atomicradius of 2.15×10⁻¹⁰ m, Ba having an atomic radius of 2.18×10⁻¹⁰ m, Yhaving an atomic radius of 1.82×10⁻¹⁰ m, Zr having an atomic radius of1.62×10⁻¹⁰ m, and Hf having an atomic radius of 1.60×10⁻¹⁰ m.

[0107] It is possible to stabilize the CeNiSi₂ type crystal structure byintroducing into the alloy at least one kind of element, M1, selectedfrom the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Nb.It should be noted, however, that, if the atomic ratio x of the elementM1 is lower than 0.5 or exceeds 1.5, it is possible for the CeNiSi₂ typecrystal structure not to be obtained. Naturally, it is desirable for theatomic ratio x noted above to be not smaller than 0.5 and not largerthan 1.5. It is more desirable for the atomic-ratio x to fall within arange of 0.6 to 1.3.

[0108] If the atomic ratio y of the element M2 is smaller than 1.5, itis possible for the ratio of the crystal phase other than the CeNiSi₂type crystal phase, e.g., the ratio of the TiNiSi type crystal phase, tobe increased. It should be noted that the lithium diffusion rate in theTiNiSi type crystal phase is lower than that in the CeNiSi₂ type crystalphase. It follows that, if the current density in the charge-dischargestage is increased in the secondary battery comprising a negativeelectrode containing an alloy having an atomic ratio y of the elementM2, which is smaller than 1.5, it is difficult to maintain a sufficientcapacity. On the other hand, if the atomic ratio y of the element M2exceeds 3.5, a phase that carries out easily the alloying reaction withlithium, e.g., the LnSn phase, is generated, with the result that thecharge-discharge cycle life of the secondary battery tends to beshortened. Such being the situation, it is more desirable for the atomicratio y to fall within a range of 1.7 to 2.5.

[0109] In the alloy having a CeNiSi₂ type crystal structure, it ispossible for a part of the constituting elements to be replaced byanother element in order to impart a local strain to the crystalstructure or to change the Fermi level of the alloy.

[0110] It is possible to prepare an alloy having the CeNiSi₂ typecrystal structure by, for example, a induction heating method. In theinduction heating method, it comprises pouring a melt onto a rotatingcooling plate in the casting stage. It is possible to control thethickness of the deposited melt by adjusting the melt supply rate andthe moving speed of the cooling plate, thereby controlling the coolingrate. In order to obtain an alloy having a CeNiSi₂ type crystalstructure, it is preferable that the melt cooling rate should be set ina range of 10¹ to 10⁴ (K/s). This is because if the cooling rate exceeds10⁴ (K/s), the crystallinity of the obtained intermetallic compoundbecomes excessively low, thus making it difficult to maintain theCeNiSi₂ type crystal structure. It is possible to apply a heat treatmentto the obtained sample so as to homogenize the texture and thecomposition of the sample.

[0111] The negative electrode can be prepared by, for example,dispersing a negative electrode mixture containing a negative electrodeactive material including an alloy having a CeNiSi₂ type crystalstructure, an electrically conductive agent and a binder in a suitablesolvent so as to obtain a suspension, followed by coating one surface orboth surfaces of a current collector with the suspension thus obtainedand subsequently drying the coating.

[0112] In the case of using as the negative electrode active material amixture containing an alloy having a CeNiSi₂ type crystal structure anda carbonaceous material having a high alkali metal absorptioncapability, it is possible to increase the absorption amount of thealkali metal such as lithium. It is desirable to use a graphitizedmaterial as the carbonaceous material acting as the negative electrodeactive material. It should be noted, however, that, if a graphitizedmaterial having a high alkali metal absorption capability is used as thecarbonaceous material, the electrical conductivity of the negativeelectrode may be lowered. Such being the situation, it is desirable touse another carbon material such as an acetylene black or a carbon blackas the electrically conductive agent together with the graphitizedmaterial used as the negative electrode active material.

[0113] It is desirable for the negative electrode containing an alloyhaving a CeNiSi₂ type crystal structure to have a structure satisfyingformula (3) given below:

0.95≧(w/d)/ρ≧0.55  (3)

[0114] where ρ denotes the true density (g/cm³) of the alloy, d denotesthe thickness (μm) of the negative electrode, and w denotes the weightper unit area (g/m²) of the negative electrode.

[0115] It should be noted that the intermetallic compound having aCeNiSi₂ type crystal structure has an inner diffusion rate of lithiumions lower than that of the graphitized material such as graphite. Itfollows that, if the intermetallic compound is mixed in a large amountwith the graphitized material, the lithium intercalation into theintermetallic compound is not completely finished even at the time whenlithium has been intercalated into the graphitized material in atheoretical capacity, with the result that the intermetallic compoundfails to exhibit its performance sufficiently. Also, if the secondarybattery containing the mixture noted above as a negative electrodeactive material is charged for a long time, the metal lithium isprecipitated on the periphery of the graphitized material so as to giverise to a problem in respect of the safety of the secondary battery. Itis possible for these problems to take place in the case where the valueof (w/d)/ρ is smaller than 0.55. It follows that it is desirable for thevalue of (w/d)/ρ to be not smaller than 0.55. It is possible to make thevalue of (w/d)/ρ very close to 1 depending on the manufacturingconditions such as the pressing pressure and the pressing rate. However,in this case, it is difficult for the electrolysis solution to permeateinside the negative electrode. Such being the situation, it is desirablefor the value of (w/d)/ρ to be not larger than 0.95. It is moredesirable for the value of (w/d)/ρ to satisfy 0.6≦(w/d)/ρ≦0.92.

[0116] Incidentally, if formula (3) given above is satisfied in respectof each of the negative electrode containing an alloy having a TiNiSitype crystal structure and the negative electrode containing an alloyhaving a ZrBeSi type crystal structure, it is possible to obtain anonaqueous electrolyte secondary battery having a high dischargecapacity per unit volume and excellent in the rate characteristics andthe charge-discharge cycle life.

[0117] The binder used in the present invention includes, for example,polyvinylidene fluoride (PVdF), a fluorinated rubber, styrene-butadienerubber (SBR) and carboxy methyl cellulose (CMC).

[0118] Concerning the mixing ratio of the negative electrode activematerial, the electrically conductive agent, and the binder, it isdesirable for the negative electrode active material to be mixed in anamount of 90 to 99% by weight, for the electrically conductive agent tobe mixed in an amount of 0 to 10% by weight, and for the binder to bemixed in an amount of 1 to 5% by weight.

[0119] The current collector used in the present invention is notparticularly limited as long as the current collector is formed of anelectrically conductive material. For example, it is possible for thecurrent collector to be formed of a foil, a mesh, a punched metal or alath metal of copper, stainless steel or nickel.

[0120] The nonaqueous electrolyte secondary battery according to thethird embodiment of the present invention described above comprises anegative electrode containing an alloy having a CeNiSi₂ type crystalstructure. In the case of using the negative electrode containing thealloy, an alloying reaction is not carried out between the alloy andlithium when lithium ions are interposed between the adjacent layers ofthe crystal in the charging stage of the secondary battery. Also, theinterposed lithium ions are released from between the adjacent layers ofthe crystal in the discharge stage of the secondary battery. It followsthat lithium can be intercalated into and deintercalated from betweenthe adjacent layers of the crystal in the charge-discharge stages of thesecondary battery without bringing about an alloying reaction betweenthe alloy and lithium so as to enhance the reversibility of theintercalation/deintercalation reaction of lithium. In addition, sincethe volume expansion can be diminished at the interposing stage oflithium, it is possible to increase the stability of the crystal. Whatshould also be noted is that it is possible to suppress the reactionbetween the negative electrode and the nonaqueous electrolyte. As aresult, it is possible to increase the Coulomb efficiency during thecharge-discharge cycle so as to achieve high charge-discharge cyclecharacteristics. The CeNiSi₂ type crystal structure of the alloy used inone embodiment of the present invention is featured in that two kinds ofelements including the elements on the Ni site, e.g., at least one kindof element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co,Ni, Cu, Zn and Nb, and the elements on the Si site, e.g., at least onekind of element selected from the group consisting of P, Si, Ge, Sn, andSb, are arranged to form a hexagonal plane. In this case, the bondingbetween the adjacent atoms is considered to be stable, with the resultthat the element on the Si site, which is coupled with lithium in thecharging stage, serves to suppress the collapse of the crystal structurewhen lithium is released from the crystal in the discharge stage. Also,since the elements on the Ni site and the Si site are arranged on asubstantially single plane, it is possible to permit the diffusion oflithium to proceed smoothly within the crystal. It follows that,according to the secondary battery comprising a negative electrodecontaining an alloy having a CeNiSi₂ type crystal structure, it ispossible to improve the charge-discharge cycle characteristics and toachieve a large discharge capacity even in the case where the currentdensity or the C rate is increased.

[0121] It is possible to permit the absorption-desorption reaction oflithium to be carried out smoothly by controlling the lattice constantof the crystal axis “a” of the alloy having a CeNiSi₂ type crystalstructure to fall within a range of 3.5 Å to 5.5 Å so as to furtherimprove the rate characteristics.

[0122] Incidentally, the alloy having a TiNiSi type crystal structuremakes it possible to perform the intercalation of lithium as describedpreviously, with the result that it is possible to achieve a largedischarge capacity per unit volume and excellent cycle characteristics.It is highly possible for the alloy of this type to achieve a highperformance under a low current density or C rate. However, the capacityis unlikely to be exhibited sufficiently under a high current density.The problem is as follows. In the TiNiSi type crystal structure, theelements on the Ni site and the Si site are arranged zigzag, resultingin failure to secure sufficiently the lithium diffusion path inside theintermetallic compound when lithium is intercalated in the intermetalliccompound by the charging. It follows that it is not possible for a highlithium diffusion rate to be obtained. Such being the situation, if thecurrent density is increased in the charge-discharge stage in thesecondary battery comprising a negative electrode containing an alloyhaving a TiNiSi type crystal structure, it is not possible for asufficient capacity to be maintained.

[0123] Each of the first to third embodiments according to the presentinvention can be applied to batteries of various shapes such as acylindrical secondary battery, a secondary battery having a rectangularcross section, and a thin plate type secondary battery. FIG. 3exemplifies the construction of the cylindrical nonaqueous electrolytesecondary battery, and FIG. 4 exemplifies the construction of the thinplate type nonaqueous electrolyte secondary battery.

[0124] As shown in FIG. 3, an insulating body 2 is arranged in thebottom portion of a cylindrical case 1 made of, for example, stainlesssteel and having a bottom. An electrode group 3 is housed in the case 1.The electrode group 3 is prepared by spirally winding a laminatestructure comprising a positive electrode 4, a negative electrode 6 anda separator 5 interposed between the positive electrode 4 and thenegative electrode 6.

[0125] A nonaqueous electrolysis solution is housed in the case 1. Aninsulating paper sheet 7 having a central open portion is arranged abovethe electrode group 3 within the case 1. An insulating sealing plate 8is fixed by caulking to the upper open portion of the case 1. A positiveelectrode terminal 9 is fitted in the center of the insulating sealingplate 8. Further, a positive electrode lead 10 is electrically connectedat one end to the positive electrode 4 and to the positive electrodeterminal 9 at the other end. On the other hand, the negative electrode 6is electrically connected to the case 1, which acts as a negativeelectrode terminal, via a negative electrode lead (not shown).

[0126] The thin plate type nonaqueous electrolyte secondary batteryshown in FIG. 4 comprises a flat plate type electrode group 11 preparedby spirally winding in a flattened shape a laminate structure includinga positive electrode 12, a negative electrode 13, and a separator 14interposed between the positive electrode 12 and the negative electrode13. A band-like positive electrode terminal 15 is electrically connectedto the positive electrode 12, and a band-like negative electrodeterminal 16 is electrically connected to the negative electrode 13. Theelectrode group 11 is housed in a case 17 formed of a laminate film suchthat the edge portions of the positive electrode terminal 15 and thenegative electrode terminal 16 protrude from within the case 17. Thecase 17 formed of a laminate film is sealed by means of a heat seal.

[0127] Incidentally, the shape of the electrode group housed in the caseis not limited to the spiral shape as shown in FIG. 3 or to a flat plateshape as shown in FIG. 4. It is also possible to prepare the electrodegroup by laminating a positive electrode, a separator and a negativeelectrode in the order mentioned a plurality of times.

[0128] Examples of the present invention will now be described in detailwith reference to the accompanying drawings.

EXAMPLES 1 TO 16

[0129] <Preparation of Positive Electrode>

[0130] In the first step, prepared was a positive electrode having anelectrode density of 3.0 g/cm³ by adding 2.5% by weight of an acetyleneblack, 3% by weight of graphite, 3.5% by weight of polyvinylidenefluoride (PVdF), and N-methylpyrrolidone (NMP) to 91% by weight oflithium cobalt oxide (LiCoO₂) powder used as a positive electrode activematerial while stirring the solution, followed by coating a currentcollector formed of an aluminum foil having a thickness of 15 μm withthe resultant mixture and subsequently drying the coating and, then,pressing the coating.

[0131] <Preparation of Negative Electrode>

[0132] For preparation of a negative electrode active material,prescribed amounts of elements were mixed at the composition ratio shownin Table 1, followed by once casting the mixture in a thickness of about10 mm on a water-cooled circular template by means of a inductionheating and subsequently applying an additional induction heating to thecast mixture so as to obtain a melt. Then, the melt thus obtained wassprayed onto a cooling roll rotated at a speed of 40 m/s so as to obtaina flake-like intermetallic compound layer having a thickness of 10 to 30μm, thereby obtaining a negative electrode active material.

[0133] In the next step, a negative electrode was prepared by adding 5%by weight of graphite used as an electrically conductive agent, 3% byweight of an acetylene black that was also used as an electricallyconductive agent, 7% by weight of PVdF, and an NMP solution to 85% byweight of the intermetallic compound powder while stirring the solution,followed by coating a current collector formed of a copper foil having athickness of 11 μm with the resultant mixture and subsequently dryingthe coating and, then, pressing the coating.

[0134] <Preparation of Electrode Group>

[0135] An electrode group was prepared by laminating the positiveelectrode noted above, a separator formed of a polyethylene porous film,the negative electrode noted above, and separator noted above in theorder mentioned, followed by spirally winding the laminate structuresuch that the negative electrode is positioned to form the outermostcircumferential surface.

[0136] <Preparation of Nonaqueous Electrolysis Solution>

[0137] Further, a nonaqueous electrolysis solution was prepared bydissolving lithium hexafluoro phosphate (LiPF₆) in a mixed solventprepared by mixing ethylene carbonate (EC) with methyl ethyl carbonate(MEC) at a mixing ratio by volume of 1:2. Lithium hexafluoro phosphatewas dissolved in the mixed solvent in an amount of 1 mol/L.

[0138] Finally, a cylindrical nonaqueous electrolyte secondary batteryas shown in FIG. 1 was assembled by housing the electrode group and thenonaqueous electrolysis solution in a cylindrical case made of stainlesssteel and having a bottom.

EXAMPLE 17

[0139] A cylindrical nonaqueous electrolyte secondary battery wasassembled as in Example 1, except that an intermetallic compoundobtained by mixing prescribed amounts of elements having a compositionratio shown in Table 1, followed by casting the resultant mixture bymeans of a induction heating method and subsequently applying a heattreatment to the cast body at 900° C. for 6 hours under an inert gasatmosphere was used as the negative electrode active material.

[0140] The intermetallic compound used in the secondary battery for eachof Examples 1 to 17 was analyzed by the X-ray diffraction method. It hasbeen found that the intermetallic compound for each of Examples 1 to 16contains a TiNiSi type crystal phase, and that the intermetalliccompound for Example 17 contains a ZrBeSi type crystal phase. Also, thelattice constant of the crystal axis b of the TiNiSi type crystal andthe lattice constant of the crystal axis “a” of the ZrBeSi type crystalwere determined from the X-ray diffraction patterns, with the results asshown in Table 1.

COMPARATIVE EXAMPLE 1

[0141] A cylindrical nonaqueous electrolyte secondary battery wasprepared as in Example 1, except that a Si powder having an averageparticle diameter of 10 μm was used as the negative electrode activematerial.

COMPARATIVE EXAMPLE 2

[0142] A cylindrical nonaqueous electrolyte secondary battery wasprepared as in Example 1, except that a mesophase pitch based carbonfiber subjected to a heat treatment at 3250° C., and having an averagefiber diameter of 10 μm, an average fiber length of 25 μm, an averagelayer spacing d₀₀₂ of 0.3355 nm, and a specific surface area asdetermined by the BET method of 3 m²/g was used as the negativeelectrode active material.

COMPARATIVE EXAMPLE 3

[0143] A cylindrical nonaqueous electrolyte secondary battery wasprepared as in Example 1, except that a LiSn alloy was used as thenegative electrode active material.

COMPARATIVE EXAMPLE 4

[0144] A cylindrical nonaqueous electrolyte secondary battery wasprepared as in Example 1, except that an FeSn₂ alloy having a hexagonalclose-packed structure and a Ni₂In type structure was prepared by a rollquenching method and the alloy thus prepared was used as the negativeelectrode active material.

COMPARATIVE EXAMPLE 5

[0145] A cylindrical nonaqueous electrolyte secondary battery wasprepared as in Example 1, except that a BeSiZr alloy having a hexagonalclose-packed structure and a Ni₂In type structure was prepared by a rollquenching method and the alloy thus prepared was used as the negativeelectrode active material.

COMPARATIVE EXAMPLE 6

[0146] A cylindrical nonaqueous electrolyte secondary battery wasprepared as in Example 1, except that a CoSn alloy having a hexagonalclose-packed structure was prepared by a roll quenching method and thealloy thus prepared was used as the negative electrode active material.

[0147] The secondary battery prepared in each of Examples 1 to 17 andComparative Examples 1 to 6 was repeatedly subjected to acharge-discharge cycle in which the secondary battery was charged to4.2V over 2 hours at 20° C. under a charging current of 1.5 Å, followedby discharging the secondary battery to 2.7V under a discharge currentof 1.5 Å at 20° C., so as to measure the discharge capacity per unitvolume. (mAh/cc) for the first cycle, and the capacity retention rate atthe 100^(th) cycle. Tables 1 and 2 show the results. Incidentally, thecapacity retention rate at the 100^(th) cycle was calculated on thebasis that the discharge capacity at the first cycle was set at 100%.TABLE 1 Lattice Discharge Capacity Composition of negative Crystalconstant capacity per unit retention rate at electrode active materialstructure (Å) volume (mAh/cc) 100^(th) cycle (%) Example 1(La_(0.4)Ni_(0.6))₄₀Sn₆₀ TiNiSi type 4.52 1680 84.3 Example 2(La_(0.4)Ni_(0.6))₈₀Sn₂₀ TiNiSi type 4.37 985 85.3 Example 3(La_(0.7)Ni_(0.3))₄₀Sn₆₀ TiNiSi type 4.94 2012 82.4 Example 4(La_(0.7)Ni_(0.3))₈₀Sn₂₀ TiNiSi type 4.78 1822 86.2 Example 5(La_(0.54)Ni_(0.46))₄₄Sn₅₆ TiNiSi type 4.73 1869 88.2 Example 6(Ba_(0.4)La_(0.2)Ni_(0.4))₇₅Sn₂₅ TiNiSi type 5.46 1034 83.1 Example 7(Nd_(0.2)Ce_(0.2)Ni_(0.6))₅₄Sn₄₆ TiNiSi type 4.62 1794 84.5 Example 8(Pr_(0.1)Y_(0.32)Ni_(0.58))₇₂Sn₂₈ TiNiSi type 4.41 1012 93.5 Example 9(Mg_(0.53)Ni_(0.47))₆₀Sn₄₀ TiNiSi type 4.03 1812 86.6 Example 10(Pm_(0.2)Hf_(0.1) TiNiSi type 4.88 1725 85.2 Sm_(0.1)Ni_(0.6))₅₄Sn₄₆Example 11 (La_(0.4)Ba_(0.05) TiNiSi type 5.13 1102 92.3Ca_(0.05)Ni_(0.5))₆₅Sn₃₅ Example 12 (La_(0.57)Nb_(0.04)Ni_(0.39))₄₃Sn₅₇TiNiSi type 4.88 1821 94.5 Example 13 (La_(0.5)Co_(0.16)Ni_(0.34))₄₄Sn₅₆TiNiSi type 4.81 1692 96.1 Example 14 (La_(0.6)Fe_(0.01)Ni_(0.39))₆₅Sn₃₅TiNiSi type 4.52 1103 94.3 Example 15 (La_(0.5)Ti_(0.2)Ni_(0.3))₄₄Sn₅₆TiNiSi type 4.92 1854 91.3 Example 16 (La_(0.52)V_(0.02) TiNiSi type4.82 1911 92.5 Ti_(0.06)Ni_(0.4))₄₂Sn₅₈ Example 17(La_(0.5)Ni_(0.5))_(66.7)Sn_(33.3) ZrBeSi type 4.65 1469.2 95.0

[0148] TABLE 2 Discharge Capacity capacity retention Composition of perunit rate at negative electrode Crystal volume 100^(th) active materialstructure (mAh/cc) cycle (%) Comparative Si — 9800 23 Example 1Comparative C — 725.04 97 Example 2 Comparative Li—Sn alloy — 3254 12Example 3 Comparative FeSn₂ Ni₂In type 1743 32 Example 4 ComparativeBeSiZr Ni₂In type 1320 54 Example 5 Comparative CoSn CoSn type 2830 43Example 6

[0149] As apparent from Tables 1 and 2, the discharge capacity per unitvolume of the secondary battery for each of Examples 1 to 17 comprisingthe negative electrode containing an intermetallic compound having aTiNiSi type or ZrBeSi type crystal structure was found to be higher thanthat for the secondary battery for Comparative Example 2 using acarbonaceous material. Also, the capacity retention rate at the 100^(th)cycle for the secondary battery for each of Examples 1 to 17 was foundto be higher than that for the secondary battery for each of ComparativeExamples 1 and 3 to 6.

[0150]FIG. 5 shows the X-ray diffraction patterns of the negativeelectrode active material under the initial state (before the charging),the charged state and the discharged state in respect of the secondarybattery for Example 5 in which (La_(0.54)Ni_(0.46))₄₄Sn₅₆ intermetalliccompound was used as the negative electrode active material. FIG. 5shows the peaks derived from the TiNiSi type crystal structure. FIG. 5also shows the X-ray diffraction pattern of the negative electrodeactive material under the charged state of the secondary battery forComparative Example 3 in which a LiSn alloy was used as the negativeelectrode active material. On the other hand, FIG. 6 shows the X-raydiffraction patterns of the negative electrode active material under theinitial state (before the charging or before the test), the chargedstate and the discharged state in respect of the secondary battery forComparative Example 6 in which a CoSn alloy was used as the negativeelectrode active material. FIG. 6 also shows the X-ray diffractionpattern of the negative electrode active material under the chargedstate of the secondary battery for Comparative Example 3 in which a LiSnalloy was used as the negative electrode active material.

[0151] As apparent from FIG. 5, the diffraction peak at about 39.8° isshifted toward the smaller angle side after the charging in thesecondary battery for Example 5, supporting that lithium wasintercalated in the charging stage into the intermetallic compoundhaving the TiNiSi type crystal structure. Also, as apparent from thediffraction pattern after the discharge, the shifted diffraction peakwas brought back to the original peak position after the discharge. Thisindicates that the crystal was shrunk while maintaining the skeletalstructure of the lattice.

[0152] On the other hand, the diffraction pattern was not shifted in thesecondary battery for Comparative Example 6. However, a peak derivedfrom the alloying reaction between Li and Sn was observed in thevicinity of 38.3° in the diffraction pattern after the charging, asshown in FIG. 6. The diffraction peak disappears after the discharge.However, it is known to the art that the generation of the LiSn alloy inthe charge-discharge stage brings about a vigorous change in the volumeof the alloy active material. It follows that the fine pulverization ofthe active material causes a deterioration of the charge-dischargecycle. In other words, it is of no difficulty to understand that theintercalation/deintercalation reaction of lithium is reversibly carriedout in the secondary battery for Example 5, and that the change involume of the intermetallic compound accompanying theintercalation/deintercalation reaction of lithium is small.

[0153] Even if the secondary battery for Example 5 was subjected to 50charge-discharge cycles, a peak in the vicinity of 38.3° derived fromthe alloying reaction between Li and Sn was not recognized in thediffraction pattern after discharge for the 50^(th) cycle and after thecharging in the 51^(st) cycle as shown in FIG. 7. On the other hand, apeak in the vicinity of 38.3°, which is derived from the alloyingreaction between Li and Sn, was not recognized in the diffractionpattern under the state after the discharge in the tenth cycle of thecharge-discharge operation, when it comes to the secondary battery forComparative Example 6, as apparent from FIG. 8. However, the peak notedabove was clearly observed in the diffraction pattern under the stateafter the discharge in the 50^(th) cycle of the charge-dischargeoperation. The experimental data suggest that the alloying reactionbetween Li and Sn takes place in the charging stage as in ComparativeExample 6 because Li is stored in the alloy as an irreversible capacityby the repetition of the charge-discharge operation, which shortens thecharge-discharge cycle life.

EXAMPLES 18 TO 31

[0154] <Preparation of Positive Electrode>

[0155] In the first step, prepared was a positive electrode having anelectrode density of 3.0 g/cm³ by adding 2.5% by weight of an acetyleneblack, 3% by weight of graphite, 3.5% by weight of polyvinylidenefluoride (PVdF), and N-methylpyrrolidone (NMP) to 91% by weight oflithium cobalt oxide (LiCoO₂) powder used as a positive electrode activematerial while stirring the solution so as to obtain a slurry, followedby coating a current-collector formed of an aluminum foil having athickness of 15 μm with the resultant slurry and subsequently drying thecoating and, then, pressing the coating.

[0156] <Preparation of Negative Electrode>

[0157] For preparation of a negative electrode active material,prescribed amounts of elements were mixed at the composition ratio shownin Table 3, and the mixture was subjected to induction heating, thusobtaining a melt. Thus obtained melt was then pour onto a rotatingcooling plate to solidify it. The cooling rate here was set to 10³(K/s). After the casting, the resultant was subjected to a heattreatment at a temperature of 900° C. for 6 hours under an inert gasatmosphere, and thus an intermetallic compound was obtained. The truedensity p of the intermetallic compound thus obtained was found to be7.25 g/cm³ as measured by the Archimedean method.

[0158] In the next step, a negative electrode was prepared by adding 4%by weight of graphite used as an electrically conductive agent, 2% byweight of PVdF, and an NMP solution to 94% by weight of theinter-metallic compound powder while stirring the solution so as toobtain a slurry, followed by coating a current collector formed of acopper foil having a thickness of 11 μm with the resultant slurry andsubsequently drying the coating and, then, pressing the coating suchthat the negative electrode thus prepared had a weight w per unit areaof 180 g/m² and a thickness d of 41.4 μm. In other words, the negativeelectrode thus prepared had a ratio (w/d)/ρ of 0.6.

[0159] <Preparation of Electrode Group>

[0160] An electrode group was prepared by laminating the positiveelectrode noted above, a separator formed of a polyethylene porous film,the negative electrode noted above, and the separator noted above in theorder mentioned, followed by spirally winding the laminate structuresuch that the negative electrode was positioned to form the outermostcircumferential surface.

[0161] <Preparation of Nonaqueous Electrolysis Solution>

[0162] Further, a nonaqueous electrolysis solution was prepared bydissolving lithium hexafluoro phosphate (LiPF₆) in a mixed solventprepared by mixing ethylene carbonate (EC) with methyl ethyl carbonate(MEC) at a mixing ratio by volume of 1:2. Lithium hexafluoro phosphatewas dissolved in the mixed solvent in an amount of 1 mol/L.

[0163] Finally, a cylindrical nonaqueous electrolyte secondary batteryas shown in FIG. 1 was assembled by housing the electrode group and thenonaqueous electrolysis solution in a cylindrical case made of stainlesssteel and having a bottom.

[0164] The intermetallic compound used in the secondary battery for eachof Examples 18 and 31 was analyzed by an X-ray diffraction method. Ithas been confirmed that the intermetallic compound used in the secondarybattery for each of Examples 18 to 31 contains a CeNiSi₂ type crystalphase. Also, the lattice constant of the crystal axis “a” of the CeNiSi₂type crystal was determined from the X-ray diffraction pattern. Table 3shows the results.

COMPARATIVE EXAMPLE 7

[0165] A cylindrical nonaqueous electrolyte secondary battery wasprepared as in Example 18, except that a Si powder having an averageparticle diameter of 10 μm was used as the negative electrode activematerial.

COMPARATIVE EXAMPLE 8

[0166] A cylindrical nonaqueous electrolyte secondary battery wasprepared as in Example 18, except that a mesophase pitch based carbonfiber subjected to a heat treatment at 3250° C., and having an averagefiber diameter of 10 μm, an average fiber length of 25 μm, an averagelayer spacing dOO₂ of 0.3355 nm, and a specific surface area asdetermined by the BET method of 3 m²/g was used as the negativeelectrode active material.

COMPARATIVE EXAMPLE 9

[0167] A cylindrical nonaqueous electrolyte secondary battery wasprepared as in Example 18, except that a LiSn alloy was used as thenegative electrode active material.

[0168] The secondary battery prepared in each of Examples 18 to 31 andComparative Examples 7 to 9 was repeatedly subjected to acharge-discharge cycle in which the secondary battery was charged to4.2V over 2 hours at 15° C. under a charging current of 1A, followed bydischarging the secondary battery to 2.5V under a discharge current of1A at 15° C., so as to measure the discharge capacity per unit volume(mAh/cc) for the first cycle. Then, the capacity retention rate at the50^(th) cycle was measured at 15° C. Table 3 shows the capacityretention rate that was obtained by the calculation on the basis thatthe discharge capacity at the first cycle was set at 100%. Also, thesecondary battery was charged to 4.2V over 2 hours at 15° C. under thecharging current of 1A, followed by discharging the secondary battery to2.5V under the discharge current of 5A. Then, the discharge capacityretention rate during the discharge at 5A was calculated on the basisthat the discharge capacity during the discharge at 1A was set at 100%.Table 3 shows the results. TABLE 3 Lattice Discharge Capacity Capacityconstant of capacity per retention retention Composition of negativeelectrode crystal axis unit volume rate of 5A rate at 50^(th) activematerial “a” (Å) (mAh/cc) discharge (%) cycle (%) Example 18LaNi_(0.5)Sn_(1.5) 4.12 1023 75.9 88.5 Example 19 LaNi_(0.5)Sn_(3.5)4.35 1832 77.5 78.5 Example 20 LaNi_(1.5)Sn_(1.5) 4.98 835 81.5 89.5Example 21 LaNi_(1.5)Sn_(3.5) 5.02 1635 82.3 83.3 Example 22LaNi_(0.8)Sn₂ 4.56 1543 79.9 85.5 Example 23 CeNi_(0.7)Si₂ 4.28 132576.7 85.3 Example 24 (La_(0.5)Ca_(0.5))(Ni_(0.5)Co_(0.3))Sn₂ 5.03 163583.5 83.2 Example 25 (Zr_(0.5)Ce_(0.5))(Ni_(1.2)Fe_(0.3))Ge₃ 3.86 153671.5 82.1 Example 26 (La_(0.7)Ba_(0.3))(Ni_(0.5)Co_(0.3))Sn₂ 5.21 133884.3 83.5 Example 27 (La_(0.7)Mg_(0.3))(Ni_(1.0)Ti_(0.3))(Sn₂P_(0.2))4.83 1476 80.7 80.5 Example 28 La(Ni_(0.3)Ti_(0.3)V_(0.3))(Si₂Sb_(0.1))4.05 1225 75.1 86.5 Example 29 La(Ni_(0.8)Cr_(0.3))Sn_(2.3) 4.53 146978.3 83.1 Example 30 Ce(Ni_(0.2)Mn_(0.3))Si_(1.8) 3.98 1054 73.2 86.7Example 31 (Ce_(0.3)Sr_(0.7))(Ni_(0.5)Zn_(0.1)Nb_(0.1))Sn_(2.3) 4.531224 79.1 83.5 Comparative Si — 9800 45.2 23 Example 7 Comparative C —725 56.1 98 Example 8 Comparative Li—Sn alloy — 3254 63.1 45 Example 9

[0169] To reiterate, the secondary battery for each of Examples 18 to 31comprised a negative electrode containing an intermetallic compoundhaving a CeNiSi₂ type crystal structure. As apparent from Table 3, thesecondary battery for each of these Examples exhibited a dischargecapacity per unit volume larger than that for the secondary battery forComparative Example 8 comprising a negative electrode containing acarbonaceous material. The secondary battery for each of the Examples ofthe present invention also exhibited the rate characteristics (capacityretention rate during the discharge under 5 Å of the discharge current)and the charge-discharge cycle characteristics (capacity retention rateat the 50^(th) charge-discharge cycle), which were superior to those forthe secondary battery for each of Comparative Examples 7 to 9.

[0170]FIG. 10 shows the X-ray diffraction patterns of the negativeelectrode active material under the initial state before the charging,the charged state and the discharged state in respect of the secondarybattery for Example 22. To be more specific, the peaks for the (200)plane and the (002) plane are shown in FIG. 10. In the secondary batteryfor Example 22, both of the (200) plane diffraction peak and the (002)plane diffraction peak were scarcely shifted after the charging, and thepeak derived from the presence of the LiSn alloy was not observed. Also,a significant change was not observed in the diffraction peaks on the(200) plane and the (002) plane after the discharge. The experimentaldata clearly support that the volume expansion scarcely occurred and theskeletal structure of the lattice was maintained. In other words, it canbe understood that the intercalation/deintercalation reaction of lithiumis reversibly carried out, and that the intercalation/deintercalationreaction of lithium is not accompanied by the change in volume of theintermetallic compound. Such being the situation, the secondary batteryfor each of Examples 18 to 31 was capable of achieving a large capacityper unit volume and a long charge-discharge cycle life.

[0171] Incidentally, a cylindrical nonaqueous electrolyte secondarybattery for Example 32 was manufactured substantially as in Example 18,except that a (La_(0.54)Ni_(0.46))₄₄Sn₅₆ alloy having a hexagonalclose-packed structure and a TiNiSi type crystal structure was preparedby a roll quenching method and used as a negative electrode activematerial. The discharge capacity per unit volume, the capacity retentionrate during the discharge under the discharge current of 5A, and thecapacity retention rate at the 50^(th) charge-discharge cycle for thesecondary battery for Example 32 were measured as in Example 18. Thedischarge capacity per unit volume was found to be 1243 mA/cc. Thecapacity retention rate during the discharge under the discharge currentof 5A was found to be 34.5%. Further, the capacity retention rate at the50^(th) charge-discharge cycle was found to be 88.5%.

EXAMPLE 33

[0172] <Manufacture of Negative Electrode>

[0173] A negative electrode was prepared by adding a graphite powder,PVdF, and an NMP solution to the intermetallic compound powder of thesame kind as that for Example 22 in accordance with the mixing ratioshown in Table 4 while stirring the solution so as to obtain a slurry,followed by coating a current collector formed of a copper foil having athickness of 12 μm with the resultant slurry and subsequently drying thecoating and, then, pressing the coating such that the negative electrodethus prepared had a weight w per unit area of 200 g/m² and a thickness dof 46 μm. In other words, the negative electrode thus prepared had aratio (w/d)/ρ of 0.6.

[0174] Incidentally, the true density p of the inter-metallic compoundused was measured by the Archimedean method by using water under anatmosphere of room temperature (23° C.).

[0175] Further, a cylindrical nonaqueous electrolyte secondary batterywas manufactured as in Example 18, except that the negative electrodeprepared as described above was used for manufacturing the secondarybattery.

EXAMPLE 34

[0176] <Manufacture of Negative Electrode>

[0177] A negative electrode was prepared by adding a graphite powder,PVdF, and an NMP solution to the intermetallic compound powder of thesame kind as that for Example 22 in accordance with the mixing ratioshown in Table 4 while stirring the solution so as to obtain a slurry,followed by coating a current collector formed of a copper foil having athickness of 12 μm with the resultant slurry and subsequently drying thecoating and, then, pressing the coating such that the negative electrodethus prepared had a weight w per unit area of 200 g/m² and a thickness dof 34.5 μm. In other words, the negative electrode thus prepared had aratio (w/d)/ρ of 0.8.

[0178] Further, a cylindrical nonaqueous electrolyte secondary batterywas manufactured as in Example 18, except that the negative electrodeprepared as described above was used for manufacturing the secondarybattery.

EXAMPLE 35

[0179] An intermetallic compound powder of the same kind as that used inExample 22 was molded into a pellet having a diameter of 1 cm, and acoin-shaped nonaqueous electrolyte secondary battery was assembled byusing the molded pellet as the negative electrode.

EXAMPLE 36

[0180] <Manufacture of Negative Electrode>

[0181] A negative electrode was prepared by adding a graphite powder,PVdF, and an NMP solution to the intermetallic compound powder of thesame kind as that for Example 22 in accordance with the mixing ratioshown in Table 4 while stirring the solution so as to obtain a slurry,followed by coating a current collector formed of a copper foil having athickness of 12 μm with the resultant slurry and subsequently drying thecoating and, then, pressing the coating such that the negative electrodethus prepared had a weight w per unit area of 200 g/m² and a thickness dof 92.0 μm. In other words, the negative electrode thus prepared had aratio (w/d)/ρ of 0.3.

[0182] Further, a cylindrical nonaqueous electrolyte secondary-batterywas manufactured as in Example 18, except that the negative electrodeprepared as described above was used for manufacturing the secondarybattery.

[0183] The secondary battery manufactured in each of Examples 33 to 36was subjected to a charge-discharge cycle test in which the secondarybattery was charged for 3 hours to 4.2V under the charging current of 1Cwith the measuring environment temperature set at 35° C., followed bydischarging the secondary battery to 3.0V at the discharge current of1C, so as to measure the negative electrode initial capacity per unitvolume and the capacity retention rate when the charge-discharge cyclewas repeated 50 times. The capacity retention rate denotes the capacityat the 50^(th) cycle on the basis that the capacity at the first cyclewas set at 100. Also conducted under the same environment was anadditional charge-discharge cycle test in which the secondary batterywas charged for 3 hours to 4.2V with the charging current of 1C,followed by discharging the secondary battery to 3.0V with the dischargecurrent of 3C. Then, the capacity retention rate after the dischargewith the discharge current of 3C was obtained from the 3C dischargecapacity on the basis that the 1C discharge capacity was set at 100%.Table 4 shows the results. TABLE 4 True PVdF Graphite Discharge CapacityCapacity density ratio in ratio in capacity retention retention ρ ofnegative negative per unit rate of 3 C rate at Alloy alloy electrodeelectrode volume discharge 50^(th) cycle composition (g/cm³) (wt %) (wt%) (w/d)/ρ (mAh/cc) (%) (%) Example 33 LaNi_(0.8)Sn₂ 7.25 2  8 0.6 154376.5 85.5 Example 34 LaNi_(0.8)Sn₂ 7.25 2  3 0.8 1654 73.2 83.2 Example35 LaNi_(0.8)Sn₂ 7.25 No No 1 1690 23.5 73.2 (pellet addition additiontype) Example 36 LaNi_(0.8)Sn₂ 7.25 5 25 0.3 763 83.2 89.3

[0184] As apparent from Table 4, the secondary battery for each ofExamples 33 and 34, which included a negative electrode having a ratio(w/d)/ρ falling with a range of 0.55 to 0.95, made it possible toimprove the discharge capacity per unit volume while maintaining a highcapacity retention rate during the discharge at 3C and a high capacityretention rate at the 50^(th) charge-discharge cycle.

[0185] On the other hand, it is considered reasonable to understandthat, in the secondary battery for Example 35 including a negativeelectrode having a ratio (w/d)/ρ of 1, the electrolysis solution failedto permeate sufficiently into the negative electrode and, thus, thecharge-discharge reaction rate was slower than the C rate when the Crate was increased, resulting in failure to obtain a sufficiently highcapacity retention rate during the discharge at 3C. Also, the secondarybattery for Example 36 including a negative electrode having a ratio(w/d)/ρ of 0.3 exhibited a sufficiently high capacity retention rateduring the discharge at 3C and a sufficiently high capacity retentionrate at the 50^(th) charge-discharge cycle and was superior to thesecondary battery for Comparative Example 8 (carbonaceous material) inthe discharge capacity per unit volume. However, the secondary batteryfor Example 36 was somewhat insufficient in the discharge capacity perunit volume, compared with the secondary battery for each of Examples 33and 34.

[0186] The experimental data support that a secondary battery having alarge capacity and stable cycle characteristics and sufficientlysatisfying the rate characteristics can be obtained by setting the ratio(w/d)/ρ to fall within a range of 0.55 to 0.95.

[0187] As described above in detail, the present invention provides anonaqueous electrolyte secondary battery having a large dischargecapacity per unit volume and a sufficiently long charge-discharge cyclelife.

[0188] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the present invention in itsbroader aspects is not limited to the specific details andrepresentative embodiments shown and described herein. Accordingly,various modifications may be made without departing from the spirit orscope of the general inventive concept as defined by the appended claimsand their equivalents.

What is claimed is:
 1. A nonaqueous electrolyte secondary batterycomprising: a positive electrode; a negative electrode containing analloy having a CeNiSi₂ type crystal structure; and a nonaqueouselectrolyte.
 2. The nonaqueous electrolyte secondary battery accordingto claim 1, wherein a lattice constant of crystal axis “a” of theCeNiSi₂ type crystal structure falls within a range of 3.5 Å to 5.5 Å.3. The nonaqueous electrolyte secondary battery according to claim 2,wherein said lattice constant falls within a range of 4 Å to 5 Å.
 4. Thenonaqueous electrolyte secondary battery according to claim 1, whereinthe alloy contains at least one kind of element selected from the groupconsisting of P, Si, Ge, Sn and Sb.
 5. The nonaqueous electrolytesecondary battery according to claim 4, wherein the alloy furthercontains at least one kind of element having an atomic radius fallingwithin a range of 1.6×10⁻¹⁰ m to 2.2×10⁻¹⁰ m.
 6. The nonaqueouselectrolyte secondary battery according to claim 1, wherein the alloyhas a composition represented by formula (A) given below:LnM1_(x)M2_(y)  (A) where Ln denotes at least one kind of elementselected from the elements having an atomic radius falling within arange of 1.6×10⁻¹⁰ m to 2.2×10⁻¹⁰ m, M1 is at least one element selectedfrom the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Nb,M2 is at least one element selected from the group consisting of P, Si,Ge, Sn and Sb, and x and y satisfy the conditions of 0.5≦x≦1.5 and1.5≦y≦3.5.
 7. The nonaqueous electrolyte secondary battery according toclaim 6, wherein the element Ln is at least one element selected fromthe group consisting of La, Ce, Pr, Nd, Pm, Sm, Mg, Ca, Sr, Ba, Y, Zrand Hf.
 8. The nonaqueous electrolyte secondary battery according toclaim 6, wherein the atomic ratio x satisfies 0.6≦x≦1.3.
 9. Thenonaqueous electrolyte secondary battery according to claim 6, whereinthe atomic ratio y satisfies 1.7≦y≦2.5.
 10. The nonaqueous electrolytesecondary battery according to claim 1, wherein the negative electrodesatisfies formula (B) given below: 0.95≧(w/d)/ρ≧0.55  (B) where ρdenotes a true density (g/cm³) of the alloy, d denotes a thickness (μm)of the negative electrode, and w denotes a weight per unit area (g/m²)of the negative electrode.
 11. The nonaqueous electrolyte secondarybattery according to claim 1, wherein the alloy is a single phase alloyor a polyphase alloy.
 12. A nonaqueous electrolyte secondary batterycomprising: a positive electrode; a negative electrode containing analloy having a TiNiSi type crystal structure; and a nonaqueouselectrolyte.
 13. The nonaqueous electrolyte secondary battery accordingto claim 12, wherein a lattice constant of crystal axis b of the TiNiSitype crystal structure falls within a range of 4 Å to 5.5 Å.
 14. Thenonaqueous electrolyte secondary battery according to claim 12, whereinthe alloy contains Sn.
 15. The nonaqueous electrolyte secondary batteryaccording to claim 12, wherein the alloy has a composition representedby formula (C) given below: (Ni_(1-(X+Z))Ln_(X)M_(Z))_(y)Sn_(100-y)  (C)where Ln denotes at least one kind of element selected from the elementshaving an atomic radius falling within a range of 1.6×10⁻¹⁰ m to2.2×10⁻¹⁰ m, M is at least one element selected from the groupconsisting of Ti, V, Co, Fe and Nb, and x, y and z satisfy theconditions of 0.4≦x+z≦0.7, 40≦y≦80 and 0≦z≦0.2.
 16. A nonaqueouselectrolyte secondary battery comprising: a positive electrode; anegative electrode containing an alloy having a ZrBeSi type crystalstructure; and a nonaqueous electrolyte.
 17. The nonaqueous electrolytesecondary battery according to claim 16, wherein a lattice constant ofcrystal axis “a” of the ZrBeSi type crystal structure falls within arange of 4 Å to 5.5 Å.